1. Field of the Invention
The present invention relates to an electrical connecting member for electrically connecting electric circuit parts, and to an electric circuit member and an electric circuit device using the connecting member.
2. Related Background Art
Hitherto, various methods have been known for constructing an electrical circuit member by electrically connecting a plurality of electric circuit components. These methods are exemplarily shown below.
(1) Wire bonding method
FIGS. 1 and 2 show a typical example of a semiconductor device having components connected and sealed by the wire bonding method. The wire bonding method will be explained hereinunder with reference to FIGS. 1 and 2.
According to the wire bonding method, a semiconductor element 4 is fixed to and supported by an element mounting portion 2 by means of, for example, an Ag paste 3. Subsequently, the bonding portions 5 of the semiconductor element 4 and the desired connecting portions 6 of a lead frame 1 are bonded through ultra-fine metallic wires 7 such as gold wires.
After the connection is completed, the semiconductor element 4 and the lead frame 1 are sealed by means of a resin 8 and, thereafter, the unnecessary portion of the lead frame 1 extending outward from the sealing resin is cut and bent, whereby a semiconductor device 9 is obtained.
(2) TAB (Tape Automated Bonding) Method (see, for example, Japanese Patent Unexamined Publication No. 59-139636)
FIG. 3 shows a typical example of a semiconductor device having elements connected and sealed by the TAB method. Briefly, this method is an automatic bonding method making use of a tape carrier system. Referring-to FIG. 3, after the carrier film substrate 16 and a semiconductor element 4 are located, the inner lead portion 17 of the carrier film substrate 16 and the connecting portion 5 of the semiconductor element 4 are heat-bonded. Thereafter, the inner lead 17 and the semiconductor element 4 are sealed by resins 20 and 21.
(3) CCB (Controlled Collapse Bonding) Method (see, for example, Japanese Patent Examined Publication No. 42-2096 and Japanese Patent Unexamined Publication No. 60-57944)
FIG. 4 shows a typical example of a semiconductor device having elements connected and sealed by the CCB method. This method, which also is referred to as the flip chip bonding method, will be explained with reference to FIG. 4. A semiconductor element 4, which is beforehand provided with solder bumps 31, is located and placed on a circuit board 32. Thereafter, the solder is heated and molten so that the semiconductor element 4 is connected to the circuit board 32. Then, after washing away the flux, the semiconductor element 4 is sealed so that a semiconductor device 9 is obtained.
(4) Method Illustrated in FIGS. 5 and 6
An insulating film 71 of, for example, polyimide is formed on the portion of a first semiconductor element 4 other than the connecting portion 5, while a metallic member 70 such as Au is placed on the connecting portion 5. Then, the exposed surfaces 73, 72 of the metallic member 70 and the insulating film 71 are flattened. Similarly, an insulating film 71' of, for example, polyimide is formed on the portion of a second semiconductor element 4' other than the connecting portion 5', while a metallic member 70' such as Au is placed on the connecting portion 5. Then, the exposed surfaces 73', 72' of the metallic member 70' and the insulating film 71' are flattened. Thereafter, the first semiconductor device 4 and the second semiconductor device 4' are located with respect to each other as shown in FIG. 6 and heat and pressure are applied to both semiconductor devices, whereby the connecting portions 5 and 5' of the first and second semiconductor elements 4 and 4' are connected to each other through the metallic members 70, 70'.
(5) Method Illustrated in FIG. 7
A first circuit board 75 and a second circuit board 75' are located with respect to each other through the intermediary of an anisotropic conductive film 78 having conductive particles dispersed in an insulating material 77. Then, pressure or pressure and heat are applied to the first and second circuit boards, whereby both circuit boards are connected at their connecting portions 76 and 76.'
(6) Method illustrated in FIG. 8
A first circuit board 75 and a second circuit board 75' are located with respect to each other through the intermediary of an elastic connector 83 composed of an insulating material 81 and metallic wires of Fe, Cu or the like unidirectionally disposed in the insulating material 81. Then, pressure is applied to both circuit boards, whereby both circuit boards are connected at their connecting portions 76, 76'.
These known bonding methods, however, suffer from the following drawbacks.
(1) Drawbacks of wire bonding method
(a) When the design is such that the connecting portion 5 of the semiconductor element 4 is within the area of the semiconductor element 4, the ultra-fine metallic wires 7, due to their extremely small diameters, tend to contact the outer peripheral portion 10 of the semiconductor element 4 or the outer peripheral region 11 of the element mounting portion 2. Such contact of the ultra-fine metallic wire 7 with the outer peripheral regions 10 and 11 causes a short-circuiting. Such a design also requires that the ultra-fine metallic wire 7 has an increased length, which in turn increases the risk for the ultra-fine wires 7 to be deformed during the transfer molding.
In order to obviate these problems, the connecting portions 5 have to be disposed on the restricted areas along the peripheral sides of the semiconductor element 4, with the result that the freedom in the circuit design is impaired.
(b) In forming a device by the wire bonding method, it is necessary that the pitch of the connecting portions 5 on the semiconductor element 4 in terms of the distance between the centers of the adjacent connecting portions must be greater than a predetermined value, in order to eliminate any possibility of interference between adjacent ultra-fine metallic wires 7. This means that the maximum number of the connecting portions 5 is essentially determined by the size of the semiconductor element 4. According to the wire bonding method, therefore, the above-mentioned pitch is as large as about 0.2 mm, so that the number of the connecting portions 5 is undesirably limited.
(c) The height h of the ridge of the ultra-fine metallic wire 7 as measured from the connecting portion 5 of the semiconductor element 4 is usually from 0.2 to 0.4 mm. It is rather difficult to reduce this height to a value below 0.2 mm. Thus, the reduction of the thickness of the whole device is difficult to achieve.
(d) Wire bonding is generally time-consuming. Thus, an impractically long time is required particularly when the number of connecting points is large, resulting in a reduction in the production efficiency.
(e) The ultra-fine metallic wire 7 tends to be deformed or, in the worst case, broken when the transfer molding condition is exceeded for any reason.
(f) At each connecting portion 5 on the semiconductor element 4 is exposed A which is not alloyed with the ultra-fine metallic wire 7. This increases the tendency for A to be corroded, with the result that the reliability is impaired.
(2) Drawbacks of the TAB Method
(a) If the design is such that the connecting portion 5 of the semiconductor element 4 is within the area of the semiconductor element 4, the length of the inner lead 17 of the carrier film board 16 is increased so that the inner lead 17 becomes liable to deform, with the result that the connection of the inner lead 17 to the desired connecting portion 5 is broken or the inner lead 17 contacts a portion of the semiconductor element 4 other than the connecting portion 5. In order to obviate this problem, it is necessary that the connecting portion 5 of the semiconductor element 4 is positioned on a peripheral portion of the semiconductor element 4, which undesirably restricts the freedom in design.
(b) The pitch of the connecting portions on the semiconductor element 4 has to be as large as 0.09 to 0.15 mm also in the case of the B method. Therefore, it is difficult to increase the number of connecting portions, as in the case of the wire bonding method as explained in (2)(b) above.
(c) In order to prevent the inner lead portion 17 of the carrier film board 16 from contacting any portion of the semiconductor element 4 other than the connecting portion, the inner lead 17 is strictly required to have a predetermined connecting form, resulting in an increased production cost.
(d) In order to connect the connecting portions 5 of the semiconductor element 4 and the inner leads 17, it is necessary that gold bumps be provided either on the connecting portions 5 of the semiconductor element or on the inner lead 17, so that the cost is raised uneconomically.
(3) Drawbacks of CCB Method
(a) Cost is raised due to the necessity of forming solder bumps 31 on the connecting portions 5 of the semiconductor element 4. A too large amount of the solder bump causes bridging between adjacent solder bumps, while a too small amount of solder bump may cause insufficient electrical connection between the connecting portion 5 of the semiconductor element 4 and the connecting portion 33 of the board 32, thus impairing the reliability of the electrical connection. In addition, the reliability of the electrical connection is influenced by the amount and shape of the solder bump. In this regard, reference is made to Technical Report of Brazing Technic Association No. 017-'84, issued from Brazing Technic Association. The fact that the amount of the solder bump affects the reliability of the electrical connection means that the amount of the solder bump has to be delicately controlled.
(c) When the solder bumps 31 are located within the area of the semiconductor element 4, it is very difficult to visually check whether the connection has been achieved in good manner.
(d) The semiconductor element exhibits inferior heat radiation characteristics. In this regard, a reference may be made to Electronic Packaging Technology 1987, 1 (Vol. 3, No. 1) p. 66-71, NIKKEI MICRODEVICES, May 1986, pp. 97-108. Various measures have to be taken for the purpose of improving the heat radiation characteristic of the product.
(4) Drawbacks of Method Illustrated in FIGS. 5 and 6
(a) The number of steps of production process is increased and the production cost is raised correspondingly due to the necessity for flattening the exposed surface 72 of the insulating film 71 and the exposed surface 73 of the metallic member 70, as well as the exposed surface 72' of the insulating film 71' and the exposed surface 73' of the metallic member 70'.
(b) The state of connection between the metallic members 70 and 70' is impaired if there is any irregularity or unevenness in the exposed surface 72 of the insulating film 71 and the exposed surface 73 of the metallic member 70, or the exposed surface 72' of the insulating film 71' and the exposed surface 73' of the metallic member 70'. In consequence, the reliability of the electrical connection is reduced.
(5) Drawbacks of Method Illustrated in FIG. 7
(a) Bonding is achieved by applying pressure to the bonding portions 76, 76' after locating two members. It is very difficult to apply this pressure uniformly on all the bonding portions. In consequence, the states of connection fluctuate greatly, with the result that the value of contact resistance also fluctuates greatly. Consequently, the reliability of the electrical connection is impaired. In addition, this bonding method is not suited to the production of devices which requires electric current to be supplied at a large rate, because the supply of such a large electric current inevitably causes a large heat generation.
(b) The fluctuation of the resistance value is inevitable due to the orientation of the conductive particles 79 of the anisotropic conductive film 78, even when the pressure is uniformalized. In consequence, the reliability of the connection is impaired, and the product produced by this bonding method cannot have a large electric current capacity.
(c) The value of the resistance between adjacent bonding portions is reduced when the pitch of the bonding portions is decreased. Thus, this bonding method is not suited to a high connecting density.
(d) The resistance value varies depending on the amount of fluctuation of the projection height h.sub.1 of the bonding portions 76, 76' of the circuit boards 75, 75'. It is therefore necessary to accurately control the amount of fluctuation of the projection height h.sub.1.
(e) When the anisotropic conductive film is used for the purpose of bonding between a semiconductor element and a circuit board or between a first semiconductor element and a second semiconductor element, a problem is encountered in that the cost is raised due to the necessity of provision of bumps on the bonding portions of the semiconductor devices, in addition to the drawbacks (a) to (d) mentioned above.
(6) Drawbacks of Method Illustrated in FIG. 8
(a) A specific pressing jig is required for applying the bonding pressure.
(b) The electric resistance across the points of contact between the metallic wire 82 of the elastic connector 83 and the bonding portion 76 of the first circuit board 75, as well as between the metallic wire 82 of the elastic connector 83 and the bonding portion 76' of the first circuit board 75', varies depending on the pressure applied and the states of the contact surfaces. Thus, the reliability of the connection is not so high.
(c) The metallic wires 82 of the elastic connector 83 are rigid. When the pressing force is large, therefore, the risk is increased for the surfaces of the first and the second circuit boards 75 and 75' to be broken. On the other hand, a too small pressing force impairs the reliability of the connection.
(d) The values of electric resistance at the contact portions, as well as the tendency for the mechanical breakdown, are largely affected by fluctuation in the amount h.sub.2 of projections of the bonding portions 76, 76' of the circuit boards 75, 75', as well as by fluctuation in the amount h.sub.3 of projection of the metallic wire 82 of the elastic connector 83. It is therefore necessary to take a suitable countermeasure for eliminating such fluctuation in the projection amounts.
(e) Drawbacks similar to (a) to (d) above are encountered also when the elastic connector is used for the bonding between a semiconductor element and a circuit board or for the bonding between a first semiconductor element and a second semiconductor element.